The present invention relates to froth flotation processes for recovery of mineral values from base metal sulfide ores. More particularly, it relates to new and improved sulfide collectors comprising certain diorganomonothiophosphinate compounds which exhibit excellent metallurgical performance over a broad range of pH values.
Froth flotation is one of the most widely used processes for beneficiating ores containing valuable minerals. It is especially used for separating finely ground valuable minerals from their associated gangue or for separating valuable minerals from one another. The process is based on the affinity of suitably prepared mineral surfaces for air bubbles. In froth flotation, a froth or a foam is formed by introducing air into an agitated pulp of the finely ground ore and water containing a frothing or a foaming agent. A chief advantage of separation by froth flotation is that it is a relatively efficient operation at a substantially lower cost than many other processes.
Current theory and practice state that the success of the sulfide flotation process depends to a great degree on reagents called collectors that impart selective hydrophobicity to the mineral value which has to be separated from other minerals. Thus, the flotation separation of one mineral species from another depends upon the relative wettability of these mineral surfaces by water. Typically, the surface free energy is purportedly lowered by the adsorption of heteropolar collectors. The hydrophobic coating thus provided, acts in this explanation, as a bridge so that the mineral particles may be attached to an air bubble. The practice of this invention is not, however, limited by this or other theories of flotation.
In addition to the collector, several other reagents are necessary. Among these are the frothing agents used to provide a stable flotation froth, persistent enough to facilitate mineral separation, but not so persistent that it cannot be broken down to allow subsequent processing. The most commonly used frothing agents are pine oil, creosote and cresylic acid and alcohols such as 4-methyl-2-pentanol, polypropylene glycols and ethers, etc.
Moreover, certain other important reagents, such as the modifiers, are also largely responsible for the success of flotation separation of the sulfide and other minerals. Modifiers include all reagents whose principal function is neither collecting nor frothing, but one of modifying the surface of the mineral so that a collector either adsorbs to it or does not. Modifying agents may thus be considered as depressants, activators, pH regulators, dispersants, deactivators, etc. Often, a modifier may perform several functions simultaneously. Current theory and practice of sulfide flotation again state that the effectiveness of all classes of flotation agents, depends to a large extent on the degree of alkalinity or acidity of the ore pulp. As a result, modifiers that regulate the pH are of great importance. The most commonly used pH regulators are lime, soda ash and, to a lesser extent, caustic soda. In sulfide flotation, however, lime is by far the most extensively used. In copper sulfide flotation, which dominates the sulfide flotation industry, for example, lime is used to maintain pH values over 10.5, more usually above 11.0 and often as high as 12 or 12.5. In prior art sulfide flotation processes, preadjustment of the pH of the pulp slurry to 11.0 and above is necessary, not only to depress the notorious gangue sulfide minerals of iron, such as pyrite and pyrrhotite, but also to improve the performance of a majority of the conventional sulfide collectors, such as xanthates, dithiophosphates, trithiocarbonates and thionocarbamates. The costs associated with adding lime are becoming quite high and plant operators are interested in flotation processes which require little or no lime addition, i.e., flotation processes which are effectively conducted at slightly alkaline, neutral or even at acid pH values. Neutral and acid circuit flotation processes are particularly desired because pulp slurries may be easily acidified by the addition of sulfuric acid, and sulfuric acid is obtained in many plants as a byproduct of the smelters. Therefore, flotation processes which do not require preadjustment of pH or which provide for pH preadjustment to neutral or acid pH values using less expensive sulfuric acid are preferable to current flotation processes, which presently require pH preadjustment to highly alkaline values of at least about 11.0 using lime which is more costly.
To better illustrate the current problems, in 1980, the amount of lime used by the U.S. copper and molybdenum industry was close to about 550 million pounds. For this industry, lime accounted for almost 92.5% by weight of the total quantity of reagents used, and the dollar value of the lime used was about 51.4% of the total reagent cost for the industry, which amounted to over 28 million dollars.
As has been mentioned above, lime consumption in individual plants may vary anywhere from about one pound of lime per metric ton of ore processed, up to as high as 20 pounds of lime per metric ton of ore. In certain geographical locations, such as South America, lime is a scarce commodity, and the current costs of transporting and/or importing lime have risen considerably in recent years. Still another problem with prior art highly alkaline processes is, that the addition of large quantities of lime to achieve sufficiently high pH causes scale formation on plant and flotation equipment, thereby necessitating frequent and costly plant shutdowns for cleaning.
It is apparent, therefore, that there is a strong desire to reduce or eliminate the need for adding lime to sulfide flotation processes to provide substantial savings in reagents costs. In addition, reducing or eliminating lime in sulfide ore processes will provide other advantages by facilitating the operation and practice of unit operations other than flotation, such as fluids handling or solids handling, as well as, the improved recovery of secondary minerals.
In the past, xanthates and dithiophosphates have been employed as sulfide collectors in the froth flotation of base metal sulfide ores. A major problem with these sulfide collectors is that at pH's below 11.0, poor rejection of pyrite or pyrrhotite is obtained. More particularly, in accordance with present sulfide flotation theory, the increased flotation of pyrite at a pH of less than 11 is attributed to the ease of oxidation of thio collectors to form corresponding dithiolates, which are believed to be responsible for pyrite flotation. Simultaneously, however, with decreasing pH, the collecting power of these sulfide collectors for copper sulfide minerals decreases, rendering them unsuitable for flotation in mildly alkaline, neutral or acid environments. This decrease in collecting power with decreasing pH, e.g., below about 11.0, requires that the collector dosage be increased many fold, rendering it generally economically unattractive. Moreover, an increase in collector dosage to promote copper sulfide flotation at these pH's further increases pyrite flotation, thereby giving rise to unacceptable copper concentrates. The decrease in copper flotation may result for many reasons. A thiol collector may interact differently with different sulfide minerals at a given pH. On the other hand, poor solution stability, i.e., the ease of oxidation, of xanthates and trithiocarbonates at lower pH values may very well explain the observed decrease in copper sulfide flotation and increase the pyrite flotation, i.e., the observed weak collector behavior.
Alkyl and aralkyl dithiophosphoric acid salts have been widely used as sulfide collectors for over 50 years. On the other hand, the organophosphorous compounds wherein there is a direct bond between C and P have not been used as widely. In U.S. Pat. No. 3,355,017, for example, it is disclosed that diisobutyldithiophosphinate may be used as the collector to provide superior collector activity, in comparison with the corresponding dithiophosphates. These collectors have, indeed, shown superior collecting properties in a variety of ores. One problem, however, is that they do not perform satisfactorily in acid, neutral or mildly alkaline circuits. Similarly, the dithiophosphates do not perform satisfactorily at a pH of less than about 10, especially for copper sulfide flotation.
In U.S. Pat. No. 2,919,025, a reagent containing a mixture of mono and dithiophosphates is disclosed to overcome the shortcomings of the dithiophosphate collectors. This reagent has been successfully used as an acid circuit collector, e.g., at pH values of from 3.5 to 6.0, for copper sulfide ores. The major drawback of this reagent is the difficulty of preparation to yield a consistent product from batch to batch. Secondly, the reagent still contains a substantial quantity of the dithio compound which exhibits poor collector strength in acid, neutral or mildly alkaline circuits. The efficacy of this reagent is therefore lowered, even though it is still better than a reagent comprising only the corresponding straight dithiophosphate. This reagent, however, did provide evidence to indicate that diethylmonothiophosphate was superior in collector behavior in an acid environment as compared with diethyl dithiophosphate.
In a related system of sulfur-containing organophosphorus compounds, the collector properties of diisobutyldithiophosphinate are known, although not as well as, those of the dithiophosphates. The collectors properties of a corresponding diisobutylmonothiophosphinate are not known in the literature. The Soviet authors, P. M. Solozhenkin, et al, in an article entitled, "Flotation Properties of Sulfur-Containing Phosphorus Derivatives," appearing in Dokl. Akad. Nauk Tadzh. SSR 13, No. 4, 26-30 (1970), disclose collector properties for diethylmonothiophosphinate in flotations of galena, pyrite and antimonite. These authors also disclose a method for the synthesis of a diethylmonothiophosphinate compound. A mention is made of diphenylmonothiophosphinic acid, but its collector property has not been studied. These authors compared the flotation collector properties of diethylmonothiophosphate/phosphinate and diethyl dithiophosphate/phosphinate in flotation of galena, pyrite and antimonite. The results of their study indicate that the dithiophosphinate is better than the monothiophosphinate at all concentrations for galena, pyrite and antimonite, and at all pH values for galena. The authors state that the increase in the flotability of materials observed upon transition from phosphorous monothio acids to dithio acids is related to the effect of electron donor substituents on an increase in the effective negative charge on the sulfur atoms responsible for the reaction with the metal cation. Their results clearly indicated that the dithiophosphinate was a much better collector than the monothiophosphinate at all pH values. The article fails to disclose or suggest the use of either diethylmonothiophosphinate or diethyl dithiophosphinate for flotation of other minerals. The article provides no trends or definite theory of the collector properties of these materials such that any prediction of their flotation properties on other minerals such as copper, for example, cannot be determined. Moreover, said article fails to disclose or suggest that selective flotation of copper sulfide minerals with simultaneous rejection of pyrite, pyrrhotite and other gangue sulfides is obtained with a diorganomonothiophosphinate at a pH of less than 10. Applicants, in contradistinction to the work of the Soviet authors, have discovered that for base metal sulfide flotation, particularly copper sulfide flotations, the monothiophosphinate compounds are far superior to the dithiophosphinates over a broad range of pH including pH's of less than about 10.0.
Accordingly, it is an object of the present invention to provide a new and improved sulfide collector and flotation process for the beneficiation of base metal sulfide minerals employing froth flotation methods which does not require preadjustment of pH to highly alkaline values.
It is another object of the present invention to provide a new and improved sulfide collector and froth flotation process for the beneficiation of base metal sulfide minerals which provides selective recovery of sulfide mineral values with selective rejection of pyrite, pyrrhotite and other gangue sulfides.
It is a further object of the present invention to provide a flotation process for the beneficiation of base metal sulfide ores at pH values of 10.0 or below using certain novel collectors containing novel donor atom combinations designed specifically for low pH flotation.
It is still another object of the present invention to provide a new and improved process for selective flotation of value sulfide minerals in acid circuits, wherein inexpensive sulfuric acid is used to control the pH.